Joining is a critical step in the fabrication of components in the aerospace industry. There are numerous techniques such as brazing, welding, mechanically forming, swaging and epoxy bonding presently available to join aircraft components together, but each of the available techniques has an attendant disadvantage for use in external aircraft assembly joinery.
For example, furnace brazing and welding techniques create at least three major disadvantages when used in external aircraft assembly joinery. The first disadvantage associated with furnace brazing and welding is that both techniques are labor intensive. The techniques are labor intensive because they both require a high degree of skill to accomplish the joinery without introducing serious defects into the joint.
The second disadvantage associated with furnace brazing and welding is that the techniques cannot satisfactorily join two dissimilar materials because of the risk of corrosion created by the metallic interaction between the two metals being joined. This inability to join dissimilar materials is a major disadvantage because the use of dissimilar materials for an external aircraft assembly can allow an aircraft assembly's design to be maximized. For example, in fluid data instruments it is desirable to construct the internal housing out of aluminum to reduce the overall component assembly weight, and to use stainless steel for the external probe end thereby providing a higher degree of corrosion and erosion resistance than if an aluminum probe end was utilized.
And the third disadvantage associated with furnace brazing and welding is that the techniques do not allow for finished parts to be joined without damage to the finished parts. The reason for the damage is because furnace brazing and welding need to introduce large quantities of heat for the techniques to be successfully employed thus degrading or destroying the finish on the finished parts that are to be joined. A look at the other present aircraft assembly techniques will also reveal the same, as well as other, limitations.
For instance, the technique of dip brazing can reduce the labor-intensive demands of the brazing and welding techniques. However, dip brazing still suffers from the limitations that the joint be made with similar materials and that the joint cannot be made using finished parts due to the amount of heat necessary to employ the technique.
Mechanical forming and swaging are plagued, just as all the aforementioned techniques, by the inability to fruitfully join finished parts. Both mechanical forming and swaging techniques will leave tool marks due to the physical contact between the finished part and the tool thereby requiring added workload to correct the marring effects of the tools. In addition, the mechanical forming technique suffers from an inherent technique limitation called spring-back. Spring-back is a result of the material's structural memory snapping back to shape after the work piece is released from the mechanical form. The consequences of spring-back are that it is difficult to achieve a strong mechanical bond and/or a sealed assembly using the mechanical forming technique.
Like mechanical forming, the technique of epoxy bonding also fails to achieve high strength attachment. Epoxy bonding also possesses other disadvantages that make it an undesirable technique for joining external aircraft components. For example, epoxy bonding does not work well on sensors equipped with heaters due to the degradation of the bonding materials by the heat source. As a result, fabrications using epoxy-bonding techniques will limit the operating temperature of an aircraft assembly. Another major disadvantage of the use of an epoxy bonding technique for external aircraft assemblies is that the epoxy bond can create an electrical discontinuity, which is not allowed due to a regulatory lightning strike requirement for external aircraft parts.
There are also alternatives to joinery such as one-piece investment casting or the use of assemblies using bolted flanges. However, both of these alternatives typically lead to heavier and more costly designs. Accordingly, there is frequently no alternative to using some type of joining process to achieve the desired configuration.
Consequently, what is needed is a method to overcome the present limitations on the manufacturing of aircraft assemblies with external applications that meet all the varying design criteria that are required. Criteria such as increased ease of fabrication, dissimilar material bonding, electrical continuity, higher strength bonding, expanded temperature capabilities, minimized mechanical stresses on parts, extended assembly life cycle, weight reduction, minimized part count and finished parts joining.